1. Field of the Invention
This invention broadly relates to a process for producing useful chemicals, and especially formaldehyde, from methane (CH.sub.4) and hydrogen sulfide (H.sub.2 S), and especially from a gas stream containing a mixture of CH.sub.4 and H.sub.2 S. More particularly, this invention provides a method wherein hydrogen sulfide, generally separated from a gas stream containing methane and hydrogen sulfide, is combined with a carbon oxide, wherein the carbon oxide is selected from carbon monoxide (CO), carbon dioxide (CO.sub.2) and mixtures thereof, and the combined gas stream is passed in contact with a catalyst comprising a supported metal oxide of a metal selected from the group consisting of titanium (Ti), zirconium (Zr), molybdenum (Mo), rhenium (Re), vanadium (V), chromium (Cr), tungsten (W), manganese (Mn), niobium (Nb), tantalum (Ta) and mixtures thereof to convert said carbon oxide and hydrogen sulfide mixture to methyl mercaptans, (primarily methanethiol (CH.sub.3 SH) and a small amount of dimethyl sulfide (CH.sub.3 SCH.sub.3)). The methyl mercaptans can be used as a starting material for making additional products and preferably are then passed in contact with a catalyst comprising certain supported metal oxides or certain bulk metal oxides in the presence of an oxidizing agent and for a time sufficient to convert at least a portion of the methyl mercaptans to formaldehyde (CH.sub.2 O) and sulfur dioxide (SO.sub.2). The carbon oxide used to react with the hydrogen sulfide preferably is recovered as a by-product of the partial oxidation of the methane into formaldehyde over a catalyst that promotes the partial oxidation of methane to formaldehyde, such as a silica supported metal oxide catalyst of a metal selected from the group consisting of vanadium (V), niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), tungsten (W), manganese (Mn), titanium (Ti), zirconium (Zr), tantalum (Ta) and mixtures thereof
2. Description of Related Art
Natural gas recovered from geological formations often contains hydrogen sulfide as an undesired impurity in concentrations of 10-30%. The hydrogen sulfide is typically separated from the methane and often is converted to elemental sulfur via the Claus Process. In the Claus Process, a first portion of the separated hydrogen sulfide is converted (oxidized) to sulfur dioxide (SO.sub.2) and the remaining portion of the hydrogen sulfide is reacted with the sulfur dioxide in the presence of a suitable catalyst to produce water and elemental sulfur. The so-produced sulfur represents a low value-added, commodity product; while the de-sulfurized methane typically is distributed for industrial and personal uses, such as for home heating.
There have been several investigations aimed at developing a process for directly converting (partially oxidizing) methane directly to the valuable chemical commodity formaldehyde. In one approach, the methane is partially oxidized over a silica-based catalyst containing a surface layer of vanadium (V), molybdenum (Mo) and the like. See Sun et al., Methane and Alkane Conversion Chemistry, pp. 219-226 (1995); Herman et al., Catalysis Today, 37:1-14 (1997) and Sun et al., J. of Catalysis, 165, 91-101 (1997). Unfortunately, the potential for commercializing the current methane partial oxidation catalyst technology is hindered by low formaldehyde yields. A sizable fraction of the methane that is consumed is converted to carbon oxide by-products. As a result, the process remains commercially uneconomical relative to the standard indirect methane-to-formaldehyde conversion process, i.e., steam reforming methane to carbon monoxide and hydrogen; water gas shifting to enhance the hydrogen to carbon monoxide ratio; hydrogenating the carbon monoxide to methanol and partially oxidizing the methanol to formaldehyde.
Ratcliffe et al., U.S. Pat. No. 4,570,020 describes a catalytic process for producing methanthiol (CH.sub.3 SH) from a gaseous feed comprising a mixture of carbon dioxide (CO) and hydrogen sulfide (H.sub.2 S). According to the patent, the gaseous mixture is contacted, at a temperature of at least about 225.degree. C. with a catalyst comprising a metal oxide of a metal selected from the group consisting of vanadium (V), niobium (Nb), and tantalum (Ta) and mixtures thereof supported as an oxide layer on titania. The methanethiol is disclosed as being useful as an odorant or tracer for natural gas and as a raw material for making methionine, fungicides and jet fuel additives.
The art has also identified methyl mercaptans, such as methanethiol (CH.sub.3 SH) and dimethyl sulfide (CH.sub.3 SCH.sub.3), as hazardous pollutants, and has suggested a variety of ways for their destruction. Noncatalytic gas phase oxidation of such reduced sulfur compounds has been shown to produce primarily sulfur oxide and carbon oxide products. A. Turk et al., Envir. Sci. Technol 23:1242-1245 (1989). Investigators have observed that oxidation in the presence of single crystal metal surfaces (Mo, Ni, Fe, Cu) results in the formation of methane and ethane, nonselective decomposition to atomic carbon, gaseous hydrogen and the deposition of atomic sulfur on the metal surface via a stoichiometric reaction (See Wiegand et al., Surface Science, 279(1992):105-112). Oxidation of higher mercaptans, e.g., propanethiol on oxygen-covered single crystal metal surfaces (Rh), produced acetone via a stoichiometric reaction at low selectivity and accompanied by sulfur deposition on the metal surface (See Bol et al., J. Am. Chem. Soc., 117(1995):5351-5258). The deposition of sulfur on the metal surface obviously precludes continuous operation.
The art also has disclosed using catalysts comprising a two-dimensional metal oxide overlayer on titania and silica supports, e.g., vanadia on titania, for catalytically reducing NO.sub.x by ammonia to N.sub.2 and H.sub.2 O in the presence of sulfur oxides. Bosch et al., Catal. Today 2:369 et seq. (1988). Thus, such catalysts are known to be resistant to poisoning by sulfur oxides. It also is known that such catalysts, as well as certain bulk metal oxides catalysts, can be used to oxidize methanol to formaldehyde selectively. Busca etal, J. Phys. Chem. 91:5263 et seq. (1987).
Applicant recently discovered that supported metal oxide catalysts, such as vanadia on titania, can be used to oxidize methyl mercaptans, such as methanethiol (CH.sub.3 SH) and dimethyl sulfide (CH.sub.3 SCH.sub.3), selectively to formaldehyde in a continuous, heterogenous catalytic process without being poisoned by the reduced sulfur. On the basis of that discovery, applicant now has envisioned the present process as a way of converting a greater portion of the methane in a sour natural gas stream to formaldehyde so as to make the direct methane-to-formaldehyde conversion process more commercially attractive.